1. Field of the Invention
The present invention relates to an optical material and an optical element, and more particularly, to an optical material suitable to form an optical element used for an image pickup optical system of a camera.
2. Description of the Related Art
Up to now, an example of methods of correcting chromatic aberration of an optical system including only a refraction system is a method using a combination of glass materials having different dispersion properties. For example, an objective lens of a telescope includes a positive lens made of a glass material whose Abbe constant (νd) is large and a negative lens made of a glass material having a small Abbe constant. A combination of the positive lens and the negative lens is used to correct axial chromatic aberration. However, the range of an optical constant of a usable lens is limited. Therefore, when a lens structure or the number of lenses is limited or when used glass materials are limited, the chromatic aberration may not be sufficiently corrected.
Therefore, in order to widen the ranges of the optical constants of the lenses, there is known a method of controlling the refractive index and the Abbe constant (νd) to obtain a glass material having a high refractive index and a low Abbe constant.
Further, U.S. Pat. No. 5,847,877 and U.S. Pat. No. 6,870,677 disclose that a heat-curable resin or a light-curable resin, which are excellent in terms of obtaining a desired shape by the application of heat or light or a thermoplastic resin for extrusion molding, have been used as an optical material including an organic compound.
When an optical element, which has an excellent chromatic aberration correction function and has an aspherical shape is to be manufactured, a case where a light-curable resin, a heat-curable resin, or a thermoplastic resin is molded on a spherical glass used as a base is more excellent in mass productivity, processability, and moldability than a case where an optical glass is used as a material. However, a plastic resin and a curable resin, which normally each have a high refractive index and a small Abbe constant, cause yellowing. That is, the resins cause yellowing because of their fundamental molecular structure, resin dissolution due to heating during processing or irradiation of energy such as ultraviolet light, or a change in molecular structure during reaction. An organic optical material used for the optical element is required to satisfy the need for high optical performance and at the same time to be more transparent.
Many optical elements obtained by molding the resin instead of the optical glass are required to have, in addition to a high transmittance and excellent optical properties, environmental reliability sufficient to obtain these properties. In particular, there is a problem that, when the optical properties are varied by the absorption of water of the molded resin or when an element shape is changed by expansion due to the absorption of water, desired element performance is not obtained. In order to solve this problem, low water-absorption resins having mainly an alicyclic skeleton have been developed and made commercially available. However, of the materials having the alicyclic molecular structure, a material satisfying various optical properties required for the optical elements has not been obtained under the current circumstances.
Meanwhile, the inventors of the present invention found that not only a high refractive index and a low Abbe constant, but also a secondary dispersion property (θg,F) are important as material properties for providing the optical element with the chromatic aberration correction function. That is, a material having a secondary dispersion property larger than a general material (refractive index extraordinary dispersion property) is very effective for chromatic aberration correction in optical design.
FIG. 1A is a graph illustrating a relationship between the Abbe constant νd and the secondary dispersion property θg,F in each of materials commercially available as optical materials. In FIG. 1A, the ordinate indicates the secondary dispersion property θg,F and the abscissa indicates the Abbe constant νd. FIG. 1B is a graph illustrating a relationship between the Abbe constant νd and the refractive index nd in each of materials commercially available as optical materials. In FIG. 1B, the ordinate indicates the refractive index nd and the abscissa indicates the Abbe constant νd.
Of the optical materials having the relationships illustrated in FIGS. 1A and 1B, examples of an optical material having a high refractive index, a low Abbe constant, and a high secondary dispersion property include UV 1000 (produced by Mitsubishi Chemical Corporation), MPV (produced by Sumitomo Seika Chemicals Co., Ltd.), and Vinylcarbazole (produced by Tokyo Chemical Industry Co., Ltd.). UV1000 has a relatively high transparent property, but water absorbency thereof is low, and thus reliability thereof is not sufficient. MPV or Vinylcarbazole is relatively excellent in water absorbency, but reliability thereof is not sufficient in view of coloring due to yellowing.